1. Field of the Invention
The present invention relates, generally, to channel-parameter estimation, and more particularly, to the estimation of channel parameters in a storage medium, such as a hard-disk drive.
2. Description of the Related Art
Magnetic media-based storage devices, such as hard-disk drives, are commonly used to store data for a wide variety of computing devices, ranging from portable music players to large-scale data centers. Improving the performance and reliability of magnetic storage devices is an ever-present goal.
A typical hard-disk drive includes a plurality of rotating platters having magnetic surfaces and a plurality of read/write heads for reading data from and writing data to the magnetic surfaces. The read/write heads are positioned relative to the magnetic surfaces using servo mechanisms and are controlled by circuitry for generating and detecting electromagnetic fields on the platters. To store data on the drive, write-channel circuitry encodes binary digital data received from a computing device into magnetic encodings, which are written onto the magnetic surfaces. To retrieve data from the drive, the servo mechanisms first locate the appropriate position, and then read-channel circuitry detects and translates the magnetic encodings at that position into the binary digital data originally stored.
The data on the platters is encoded using magnetic-flux reversals between two adjacent locations on the platter in a scheme known as “non-return-to-zero” (NRZ), whereby a bit having a value of 1 is stored on the disk as magnetization in one direction and a bit having a value of 0 is stored as magnetization in the opposite direction.
In one data-detection technique known as peak detection, which was used in early days of longitudinal recording approach, the read channel detects the locations where magnetization changes direction, enabling the recovery of stored sequences of binary ones and zeros. However, this scheme relies on magnetic transitions being relatively isolated from one another, which limits storage density and, consequently, the storage capacity of the drive.
In another data-detection technique known as partial-response maximum-likelihood (PRML) detection, an analog “playback voltage” is digitally sampled to determine the bit pattern most likely represented by the analog waveform sensed by the read head. PRML improves storage density relative to peak detection by writing magnetic transitions closer to one another than in the peak-detection method. Although the reduced isolation of adjacent magnetic transitions tends to cause inter-symbol interference (ISI) due to the overlap of information pulses, PRML detection takes ISI into account by employing equalization techniques to shape the signal from the read head into a target polynomial characterizing the extent to which each bit overlaps with adjacent bits. Accordingly, in storage devices employing PRML detection, read-channel circuitry compensates for such ISI.
If no nonlinearities exist in the recording system, then the playback voltage V(t) in a hard-disk drive can be described by the equation:
            V      ⁡              (        t        )              =                  ∑        k                                      ⁢                          ⁢                        1          2                ⁢                  (                                    a                              k                +                1                                      -                          a              k                                )                ⁢                  h          ⁡                      (                          t              -              kT                        )                                ,where akε{−1, +1} represents the kth NRZ write-current bit, t represents continuous time, T represents the duration of one bit, and h(t) represents the playback voltage response to an isolated transition.
In a typical hard-disk drive, however, a variety of nonlinearities exist in the recording system, for which the read/write channel should compensate. Improved accuracy in characterizing and compensating for these nonlinearities results in enhanced hard-disk drive performance.
A research paper containing background information concerning such nonlinearities is Palmer et al., “Identification Of Nonlinear Write Effects Using Pseudorandom Sequences,” IEEE Transactions On Magnetics, Vol. Mag-23, No. 5, September 1987, pp 2377-2379, which is incorporated herein by reference in its entirety and which discusses linear and nonlinear distortions that occur in read channels and describes a technique for separating linear and nonlinear effects, based on the unique properties of a maximal-length pseudorandom bit sequence (PRBS; also referred to as an “m-sequence”). Palmer et al. describe a technique known as “dibit extraction,” which analyzes a “dibit response” (i.e. response of read-head to an isolated NRZ bit) to identify and examine nonlinear distortion effects.
Dibit extraction is based on writing and then reading an integer number of periods of a maximal-length PRBS sequence and extracting a dibit response from the playback voltage V(t), whereby the NRZ data bits are deconvolved from the playback data (i.e. a process of determining the system transfer function or impulse response from measured outputs of the system and information on the input) to obtain the dibit response. It is noted that the term “dibit” is used in the art to refer both to the dibit itself and to the dibit response.
If the dibit response is accurate, then proper compensation for nonlinearities in the read channel can be made, e.g., by adjusting the signal as it is being written to disk by using write pre-compensation, or alternatively, by estimating channel pulse-width parameters to improve read operations. The extracted dibit contains information about several types of nonlinear distortions. One type of nonlinear distortion is a nonlinear transition shift (NLTS), which involves two transitions written closely enough that the demagnetizing fields from the previous transitions affect the timing of the writing of the next transition. Another type of nonlinear distortion is a hard transition shift (HTS), which involves reversing the direction of magnetization in already magnetized areas. Other types of nonlinear distortions include, e.g., partial erasure, magneto-resistive head asymmetry (MRA), and overwrite (OW). Either the extracted dibit itself or certain metrics computed using the extracted dibit can be used to adapt recording channel parameters to minimize such nonlinearities.
In a typical application, a maximal-length PRBS sequence (e.g., a 127-bit PRBS sequence) is written to disk and then read back to obtain a playback voltage V(t). If ak is a bit in a maximal-length NRZ PRBS sequence, then nonlinear effects are associated with various products of the bits. For example, MRA is associated with akak−1, and NLTS is associated with ak−1akak+1. Based on the “shift and add” property of maximal-length PRBS sequences (in which the modulo-2 addition of any two identical PRBS sequences with different phases generates another identical PRBS sequence, but with another phase), the nonlinear NRZ bit products produce echoes in the extracted dibit.
Such nonlinear echoes often cause interference with one another or with the main dibit response, resulting in constructive or destructive interference that can adversely affect the accuracy and usefulness of dibit extraction for characterizing the read/write channel.
Accordingly, it is important to determine the locations of these echoes and compensate for them appropriately.
FIG. 1 shows an exemplary waveform illustrating a dibit response with echoes due to nonlinearities in a longitudinal-recording scenario. FIG. 1 shows the main dibit response 101 along with some possible nonlinear echoes 103, 104, and 106. The vertical axis represents the amplitude of the dibit response signal in volts, and the horizontal axis represents the discrete time shifts of the PRBS sequence. In FIG. 1, 127 values are spread across the horizontal axis in a range of discrete time shifts (measured in bits) from −63 bits to +63 bits. The main dibit response 101 represents the linear response of the read channel. The location of each echo is a result of the “shift and add” property of the PRBS sequence interacting with a particular nonlinearity, and it is therefore possible to predict, generally, the location of each echo. Although, in a longitudinal-recording scenario, the main dibit response 101 has a dipulse shape, as shown in FIG. 1, it is noted that the main dibit response in a perpendicular-recording scenario (not shown) would have the shape of a Gaussian-like pulse.
One known method for estimating channel parameters (e.g., NLTS) involves taking the ratio of associated echo amplitude (or echo area) to the amplitude of the main pulse of the dibit. However, this method results in inaccurate estimation, because there may be overlapping echoes due to more than one type of distortion.